Deep and intermediate-depth non-double couple earthquakes: interpretation of moment tensor inversions using various passbands of very broadband seismic data

Abstract

SUMMARY Analysis of moment tensor inversions using various passbands of very broadband seismic data provides clear evidence that some non-double couple moment tensors of intermediate-depth and deep earthquakes result from the superposition of different double couple sources whose predominant principal axes are aligned with the predominant strain state within the subducting slabs. The analyses are performed for three intermediate-depth and deep earthquakes: 1984 January 1 south of Honshu (386 km), 1985 April 23 Luzon (181 km), and 1987 May 7 northern Sea of Japan (417 km). Consistent non-double couple moment tensors obtained using different sets of seismic waves in various low frequency bands suggest that these three significant non-double couple components are not caused by unmodelled propagation errors of seismic waves in the inversion procedure, because the various sets of seismic waves traverse very different ray paths. For the south of Honshu and the Luzon events, two major arrivals in the broadband P-wave diplacement seismograms are observed, with varying relative amplitudes or polarities from station to station. The different double couple mechanisms which model the two phases combine to produce the significant non-double couple moment tensors obtained at long periods. The principal axes of the subevents, closest in orientation to the predominant strain states within the slabs, tend to be quite stable, whereas the other two principal axes rotate between subevents. This observation may explain the global nature of non-double couple components in relation to the strain regime within the slab. For the northern Sea of Japan event, two major phases are observed in the P-wave displacement waveforms, but the variation in mechanism of two subevents that we model is not enough to explain the large non-double couple component observed at long periods. However, since the non-double couple component is exceptionally large compared with those for other deep earthquakes, changes of focal mechanism with shorter delays than we can resolve appear to explain the overall radiation most simply.